Skeletons don't actually weigh that much, yes there is differences some people have lighter thinner bones than others, but it's still not a major differentiator.
I'd guess the main difference is muscle mass. Someone who is twelve stone and a size ten at this height I would assume has more lean muscle and less fat than someone who ten stone and a size ten.
I tnink also frame has something to do with dress size. For example I'm very narrow even though I'm nearly five eight, my whole frame is narrow, from shoulders to hips, and when I was pregnant even though I had an emergancy c section, the consultant already said at 36 weeks they didn't think I could give birth naturally as my baby would get stuck on the way down, the bones were too close together for her head to pass, there was simply not enough space.
My daughter is the same, she is also very narrow and long. Howver I have a friend who is much much thinner than me, and the same height but she takes the same dress size, for the simple reason that her frame is simply wider, her shoulders, hips etc. But she is thinner and she looks thinner, it's simply I have a narrower frame.